Geolocation distance between two points with lat and lon values

haversine.php

<?php

/**
 * https://www.movable-type.co.uk/scripts/latlong.html
 * @param $lat1
 * @param $lat2
 * @param $lon1
 * @param $lon2
 *
 * @return float|int
 */
function _haversine($lat1, $lat2, $lon1, $lon2) {
  $R = 6371; // earth radius in km
  $fione = $lat1 * pi() / 180; // φ, λ in radians
  $fitwo = $lat2 * pi() / 180;
  $deltafi = ($lat2 - $lat1) * pi() / 180;
  $deltadelta = ($lon2 - $lon1) * pi() / 180;

  $a = sin($deltafi / 2) * sin($deltafi / 2) +
    cos($fione) * cos($fitwo) *
    sin($deltadelta / 2) * sin($deltadelta / 2);
  $c = 2 * atan2(sqrt($a), sqrt(1 - $a));

  $d = $R * $c; // in km

  return $d;
}